Splinter is the adoptive father of the turtles in the Teenage Mutant Ninja Turtles franchise.
Splinter is a mutated rat and the turtles’ martial-arts teacher, guardian, and spiritual guide. He raises Leonardo, Donatello, Raphael, and Michelangelo in the sewers and trains them to defend themselves and protect others.
His exact origin differs by continuity. In many versions, Splinter is Hamato Yoshi transformed into a rat or is a rat who learned martial arts by observing Yoshi. The 1987 cartoon and several later adaptations use different details, so Hamato Yoshi is not always literally the same individual as Splinter.
That variation explains a common mix-up. Splinter is the name used for the turtles’ father figure, while Hamato Yoshi is a human identity or backstory element in many versions. His relationship with Shredder also changes between adaptations, but his role as mentor remains central.